As mentioned previously, Graham Mancha, in Great Britain, has spare parts, including the acrylic seats & backs of the Plia chair. Right now he states he has the solid black (ABS plastic) ones too.
http://www.mancha.demon.co.uk/spares.html
I have not been able to find anyone else who carries Plia parts. I have purchased the chair glides from him, & everything went very well.
Plia-lover
August 2010